Recently, based on a WDM (wavelength Division Multiplexing scheme) which may correspond to a rapid increase of the Internet traffic, an OTN (Optical Transport Network) has been recommended in an ITU-T (International Telecommunication Unit Telecommunication standardization sector) as a so-called transparent platform where an upper layer may not have to recognize a lower layer when a client signal is transmitted from end to end not only in a synchronous network such as an SDH (Synchronous Digital Hierarchy), a SONET (Synchronous Optical NETwork) and the like but also in an asynchronous network such as an IP (Internet Protocol) or the Ethernet based network and the like. The interface and the frame format of the OTN are standardized in recommendation G. 709 of the ITU-T, and have been rapidly introduced in various commercial systems.
FIG. 1 schematically illustrates an example configuration of a SONET transmission system including transmission devices. As illustrated in FIG. 1, in the SONET transmission system, the transmission devices 1A, 1B, 1C, and 1D are connected to each other forming a ring network having a redundant configuration including a working line (Work) (i.e., a currently working line) and a backup line (Protection) described in a solid line and a dotted line, respectively. In the working line (Work) and the backup line (Protection), data are transmitted in the clockwise and counterclockwise directions, respectively. Further, the transmission devices 1B, 1C, and 1D operate in synchronization with a master clock of the transmission device 1A.
FIG. 2 illustrates an example configuration of transmission devices in the SONET transmission system. In the configuration of FIG. 2, a signal which is input from an interface on the client side is terminated by a SONET frame synchronizer 2, and then the clock of the signal is replaced by a system clock from a clock generator 4 in a clock transfer stuff generator 3. Then, the signal is mapped to a SONET frame by a SONET frame generator 5 and is output to a network via a network-side interface.
In the same manner, an input signal from the interface on the network side is terminated by a SONET frame synchronizer 6, and then the clock of the signal is replaced by the system clock from the clock generator 4 in a clock transfer stuff generator 7. Then, the signal is mapped to a client frame by a client frame generator 8 and is output to a client transmission path via a client-side interface.
The network-side interface has a redundancy configuration including the working line (Work) and the backup line (Protection), such that when a failure occurs in the working line (Work), the data transmission line is switched from the working line (Work) to the backup line (Protection) so as to maintain the transmission of the signal (data).
The operation of switching the data transmission line in the redundancy configuration (redundant switching operation) of the SONET transmission system is described. Namely, the operations when the working line in the SONET transmission system of FIG. 2 is cut and the system switches to the backup line are described with reference to a time chart of FIG. 3.
In FIGS. 2 and 3, the symbol (A) denotes a state of the working line on the network side. The symbol (B) denotes an alarm detection result of the working line on the network side. The symbol (C) denotes a selection signal selecting either the working line or the backup line on the network side. The symbol (D) denotes a state of the selected line. The symbol (E) denotes a switching signal of an AIS (Alarm Indication Signal) reporting an occurrence of a failure (an error) to an upper layer. The symbol (F) denotes an output signal of an interface on a client side.
In the time chart of FIG. 3, at time T1, a failure of the line cut or the like occurs in the working line. Then, at time T2, the SONET frame synchronizer 6 on the network side detects an alarm indicating the line cut (a line cut alarm). Further, at time T3, the system is switched from the working line to the backup line in response to the line cut alarm.
The time required for the redundant switching operation is described below. The time for detecting the line cut alarm (alarm detection time, i.e., between T1 and T2) is approximately 1 ms or less. The time for controlling to switch the line (switch control time, between T2 and T3) is approximately 40 ms or less. Therefore, it is realized that the time required for the redundant switching operation is approximately 50 ms or less which may meet a requirement in the switching time for the network system in the above redundant switching operation.
In the SONET transmission system, a synchronization clock is used in the entire network, so that the synchronization clock is used for signal processings in the signal processors in the devices of the system. On the other hand, the OTN transmission system is applied to an upper layer of the SONET transmission system and is similar to the transmission path of the WDM system. Therefore, it may be necessary to transparently transmit a signal from a client interface (as the transparent platform).
Further, the client interface may have to serve as a SONET/SDH interface and may be necessary to transmit client signals having various transmission rates due to various platforms such as the Ethernet (registered trademark), a fibre channel and the like. Therefore, the client signal and the network signal are asynchronously processed. As a result, when the client signal is transmitted as the network signal, the frequency component of the signal is transmitted as information, so that the receiver side reproduces the client interface signal based on the received frequency component.
FIG. 4 illustrates an example network configuration of the OTN transmission system. As illustrated in FIG. 4, transmission devices 1A, 1B, 1C, and 1D are connected to each other forming a ring network having a redundant configuration including a working line (Work) (i.e., a currently working (using) line) and a backup line (Protection) described in a solid line and a dotted line, respectively. In the working line (Work) and the backup line (Protection), data are transmitted in the clockwise and counterclockwise directions, respectively. Further, the transmission devices 1A, 1B, 1C, and 1D operate asynchronously.
FIG. 5 illustrates an example configuration of the OTN transmission system. As illustrated in FIG. 5, a client signal from a client transmission path is converted into an electric signal by an O/E (Optical/Electronic) converter 21, and the client clock is extracted by a client interface 22. Then, the client signal is transmitted to an ODU frame generation stuff generator 23. The ODU frame generation stuff generator 23 maps the client signal to an ODUk frame. In this case, JC bytes which are stuff information as frequency adjustment information of the client signal are added to an overhead of the ODUk frame, and stuff bytes for absorbing the fluctuation in a time domain of the client signal are inserted into a payload area or an overhead area of the ODUk frame.
The ODUk frame output from the ODU frame generation stuff generator 23 is mapped to an internal frame by an internal frame generation stuff generator 24. The internal frame is transmitted through a cross connector and a multiplex separator (which are not shown) and terminated by an internal frame synchronization stuff terminator 25 so as to be the ODUk frame. A clock generator 26 generates the system clock and supplies the system clock to the ODU frame generation stuff generator 23, the internal frame generation stuff generator 24, the internal frame synchronization stuff terminator 25 and the like.
Further, the overhead and FEC (Forward Error Correction) are added to the ODUk frame by OTU frame generators 27A and 28B for the working line and the backup line, respectively, so as to become OTUk frames. The OTUk frames are converted into optical signals by E/O (Electronic/Optical) converters 28A and 28B, and transmitted to an OTN network.
On the other hand, the OTU signal from the working line of the OTN network is converted into an electronic signal by an O/E (Optical/Electronic) converter 31A and terminated by an OTU frame synchronizer 32A so as to become an ODUk frame and be supplied to a selector (SEL) 35. A clock generator 33A generates the clock extracted from the OTU signal and in synchronization with the network clock, and supplies the generated clock to the OTU frame synchronizer 32A, an internal frame generation stuff generator 36 described below, and the like. Similarly, the OTU signal from the backup line is converted into an electronic signal by an O/E (Optical/Electronic) converter 31B and terminated by an OTU frame synchronizer 32B so as to become an ODUk frame and be supplied to the selector (SEL) 35.
The ODUk frame selected by the selector 35 is mapped to the internal frame by an internal frame generation stuff generator 36. In this case, stuff to absorb a time-domain fluctuation of the OTUk frame in the network is generated and inserted into the internal frame. The internal frame is transmitted through a cross connector and a multiplex separator (which are not shown) and terminated by an internal frame synchronization stuff terminator 37 so as to be the ODUk frame.
The ODUk frame is supplied to an ODU frame synchronization stuff terminator 38, by which data, a clock, and a write enable signal are extracted, so that the extracted data, clock and the write enable signals are written into a clock transfer memory 39. Further, the system clock generated by the clock generator 26 is supplied to the internal frame generation stuff generator 36, the internal frame synchronization stuff terminator 37, the ODU frame synchronization stuff terminator 38 and the clock transfer memory 39.
The ODU frame synchronization stuff terminator 38 specifies the insert position of a stuff byte based on stuff information (JC byte) extracted from the overhead of the ODUk frame, prevents the writing in the overhead area and the stuff byte, and generates a write enable signal instructing (causing) the writing in a data part of the payload area. Therefore, the write enable signal output from the ODU frame synchronization stuff terminator 38 is based on the transmission rate (i.e., the stuff information) of the client signal in the transmission device on the transmission side.
Further, the write enable signal output from the ODU frame synchronization stuff terminator 38 is supplied to a PLL (Phase Lock Loop) 40. As the read clock, the PLL 40 generates a clock in synchronization with the write enable signal and by smoothing the client signal. The PLL 40 supplies the generated read clock to the clock transfer memory 39 and a client transmission interface 41.
Based on the read clock, the data of the client signal are read from the clock transfer memory 39 and are output as the client signal from the client transmission interface 41. The client signal is transmitted through a selector (SEL) 42 and converted into an optical signal by an E/O (Electronic/Optical) converter 43 to be transmitted to the client transmission path.
The alarm signals indicating the line cut or the like from the O/E converter 31A and detected by the OTU frame synchronizer 32A for the working line are transmitted to a switch controller (SW CONT) 45 via an OR circuit 34A. Similarly, the alarm signals indicating the line cut or the like from the O/E converter 31B and detected by the OTU frame synchronizer 32B for the backup line are transmitted to the switch controller 45 via an OR circuit 34B. Under the control by the switch controller 45, the selector 42 selects either the client signal output from the client transmission interface 41 or an AIS (Alarm Indication Signal) generated by an AIS generator 44, and supplies the selected signal to the E/O converter 43.
The redundant switching operation of the OTN transmission system is described. Namely, the operations when the working line in the OTN transmission system of FIG. 5 is cut and the switching to the backup line are described with reference to a time chart of FIG. 6. In FIGS. 5 and 6, the symbol (A) denotes a state of the working line on the network side. The symbol (B) denotes an alarm detection result of the working line on the network side. The symbol (C) denotes a first control signal output from the switch controller 45 to select either the working line or the backup line on the network side. The symbol (D) denotes a state of the selected line. The symbol (E) denotes a second control signal output from the switch controller 45 so as to transmit (report) the AIS reporting an occurrence of a failure (an error) to the upper layer. The symbol (F) denotes an output signal of the interface on the client side. The symbol (G) denotes the operations of the PLL 40.
In the time chart of FIG. 6, at time T11, a failure of the line cut or the like occurs in the working line on the network side. Then, at time T12, the OTU frame synchronizer 32A on the network side detects the alarm indicating the line cut (a line cut alarm). Further, at time T13, in this case, the switch controller 45 switches from the working line to the backup line based on the line cut alarm. At time T14, a clock extraction of the PLL 40 (pulling in the clock to the PLL 40) is completed.
The time required for the redundant switching operation is described below. The time for detecting the line cut alarm (alarm detection time, i.e., between T11 and T12) is approximately 1 ms or less. The time for the control to switch lines (switch control time, between T12 and T13) is approximately 40 ms or less. On the other hand, the maximum clock extraction time by the PLL 40 (between T13 and T14) is approximately 3 s. The reason why the clock extraction time by the PLL 40 (between T13 and T14) requires such a longer time is that the PLL 40 loses synchronization in the switch control time between (T13 and T14).
On the other hand, there is a proposed technique (e.g., Japanese Laid-open Patent Publication No. 10-285081) to prevent the clock of the PLL in the demodulator being out-of-synchronization. To that end, in normal state, a monitor pilot signal “a” having a frequency of the intermediate value between the highest and the lowest frequencies among the input signals b1 through N is transmitted. When a specific working line “c” is degraded, a switching signal is transmitted from the demodulator to a receiving terminal switch controller. The receiving terminal switch controller transmits a transmission terminal switching signal to a transmission terminal switch controller.
The transmission terminal switch controller synchronizes the frequency of the pilot signal “a” with the frequency of the signal frequency of the degraded working line “c”, and then, performs a wireless transmission by transmitting the input signal of the working line to the backup line as well. As a result, in the demodulator, when the signal is changed from the pilot signal “a” to an input signal “b1”, since the frequency of the pilot signal “a” is the same as that of the input signal “b1”, the clock synchronization of the PLL in the demodulator may be maintained, and the switching operation of a non-interruption switcher may be surely performed based on a signal from the receiving terminal switch controller.
The OTN transmission system may be considered as an upper layer of the SONET transmission system. Therefore, the redundant switching operation of the OTN transmission system may be required to be completed within 50 ms, similar to that of the SONET transmission system. To that end, it may be necessary to reduce the time for extracting the clock by the PLL (i.e., the time period to extract (pull in) the clock by the PLL) (hereinafter may be referred to as “pull-in time” or “(clock) extraction time”). Generally, in the characteristics of the PLL, there is a trade-off relationship between the pull-in time (extraction time) and the output jitter. Namely, the output jitter is likely to increase as the pull-in time is decreased.
In the OTN transmission system, various types of client signals are multiplexed and separated. Therefore, when the signals are multiplexed and separated, extra signals are inserted and removed based on the frequency components of the client signal and the OTN transmission system (i.e., a stuff processing is performed). Therefore, the PLL may be required to have jitter suppression characteristics and, as a result, the pull-in time of the PLL may be longer. Due to the requirement of the jitter suppression characteristics, it may become necessary for the PLL to have the cut-off frequency of 1 Hz or less. In this case, the pull-in time becomes approximately 3 s. As a result, it may become difficult to reduce the switching time to 50 ms or less.